1. Field of the Invention
This invention relates in general to a method of reducing the loss of silicide and more particularly to a method of reducing the loss of metal silicide in pre-metal etching by adding a thermal process before pre-metal etching.
2. Description of Related Art
As the integrity of integrated circuit devices increases, e.g. the density of the integrated circuit increases, the characteristics of the devices such as its operating speed can be seriously effected. This is because the resistances of the source and drain electrodes of metal oxide semiconductor (MOS) transistors of the devices are gradually increased to be equal to the channel resistance of the MOS transistors. Therefore, the sheet resistance of the source and drain electrodes must be reduced by forming a low resistance metal silicide on the surface of the source and drain electrodes or on the surface of the polysilicon of the gate, and to ensure the completeness of the shallow junction between the metal and the MOS transistors.
When the low resistance metal silicide is formed, the MOS transistors are completed. After that, a glass layer is formed for protection and isolation. Since the glass transition temperature of the glass layer is low, the glass layer can be planarized by heating to cause the glass layer to flow. After forming contact windows by photolithography and etching processes, the glass layer can be reflowed by heating, for better deposition of metal sputtering in the defined contact windows. Before metal sputtering, a pre-metal etching is used to remove native oxide on the surface of the metal silicide.
Since pre-metal etching will also etch the metal silicide, the metal silicide layer becomes too thin. This results in a significant increase of the sheet resistance of the source/drain electrodes and the junction leakage.
FIG. 1 is a flow chart illustrating the conventional processes before metal sputtering. After forming the protecting glass layer, the contact windows for the ohmic contact between the metal layer and metal silicide must be defined, which is done by dry etching step 10. The photo and photoresist stripping steps are not shown in the figure. Since the metal silicide will react with oxygen in the ambient atmosphere to form native oxide layer, a pre-metal etching step 12 is conducted to remove the native oxide layer before metal sputtering.
FIGS. 2A and 2B are schematic diagrams illustrating the conventional process before metal sputtering and the conventional process after metal sputtering. In FIG. 2A, before pre-metal etching step, the main structure of the MOS transistor is completed, which includes a silicon substrate 20, and a polysilicon gate 22 and doped source/drain electrodes regions 24 formed on the silicon substrate. A metal silicide layer 26 made of TiSi.sub.2 is formed on the polysilicon gate 22 and doped source/drain electrodes regions 24. A glass protecting layer 28 is deposited thereon. After dry etching, contact windows are defined, but the metal silicide layer will be damaged and a damaged metal silicide layer 25 thereby formed.
Referring to FIG. 2B, after pre-metal etching, the damaged metal silicide layer 25 is easily removed. Thus, the metal silicide layer 29 left in the contact window will become shallow. As a result, the resistance of the metal silicide layer 29 will be seriously increased, and the junction leakage and sheet resistance of the source/drain electrodes will also increase. Table I below shows the resistance change of the metal silicide.
TABLE I ______________________________________ resistance after duration of resistance after original resistance dry etching pre-metal etching pre-metal etching ______________________________________ 3.3 .OMEGA. 4.77 .OMEGA. 20 sec 25.4 .OMEGA. 3.3 .OMEGA. 4.77 .OMEGA. 40 sec 225.2 .OMEGA. ______________________________________